(1) Field of the Invention
The invention relates to devices for measuring the geometry of defects that have altered the surface of an electrically conducting pipe.
(2) Description of the Related Art
Pipelines and other structures used in the petrochemical industry are generally made of steel, and are pressurized. Defects such as corrosion, gouges, cracks, or other imperfections or features that remove or alter a portion of the steel can affect the ability of the pipe or structure to operate safely. When a defect or imperfection is discovered it must be assessed using engineering techniques defined in the operating code covering the installation, or by using methods developed and approved by the operator. This assessment requires a calculation using the material properties and design standards of the original construction, and measurements of the defect or imperfection.
Defect assessment in high pressure pipes is typically performed for corrosion, gouges or metal loss defects, and typically follows one of three industry-accepted methods. The most common is B31G (ASME B31G, Manual for Determining the Remaining Strength of Corroded Pipelines). This assessment technique requires the length and maximum depth of a metal loss or corrosion defect, which, with pipe material information and original design standards, can be used to calculate a safe operating pressure for the defect.
More complex assessment methods can be used to minimize unnecessary repairs. Using modified material properties and a different shape factor, a modified assessment can be made that is less conservative than the original B31G. This is known as the 0.85 dt method. This technique also requires that the length of the defect or imperfection and the maximum depth of the defect or imperfection be known. These measurements are used to calculate a safe operating pressure for the defect or imperfection.
The assessment technique with the least variability is the exact or effective area technique. This technique uses the exact cross-sectional area of the defect. The area is determined from an axial depth profile which uses the maximum corrosion depth at each axial measurement spacing. This axial depth profile is projected to a linear representation of the defect, and then the area of metal loss is calculated.
Each of these methods is defined in detail in the appropriate code. Each method specifies the defect measurements required to make the calculation to determine the effect of the defect or imperfection on the safety of the pipeline or installation.
Regardless of which assessment method is used, the input data are usually provided by local measurements on the outside of the pipe or structure.
The simplest case is that of a single isolated corrosion pit or area of metal loss. A scale measures the length of the defect area. A dial extension gage (pit gage) is placed over the pit (assuming the base will span the pit) and the maximum depth read and recorded. Slightly more complicated is the case of several overlapping pits or metal loss areas or a small patch of corrosion. In such cases, the length can still be measured with a scale. An attachment, such as a bridging bar, often spans the entire defect or imperfection, providing a reference surface from which to measure depth. It is not always possible to readily locate the deepest pit or metal loss within the grouping from a visual examination, so several independent depth measurements must be taken. It is also difficult to determine if defects in close proximity interact as defined by the rules outlined in the appropriate codes.
When corrosion or metal loss is extensive and an exact area assessment is needed, it is essential that the defect be accurately mapped to form a contour plot. In these cases, a rectangular grid is drawn or painted on the pipe or structure surface, including the corroded or metal loss area. Depth measurements are taken at each grid intersection. From this array of measurements, either manual or computer-aided processing is used to construct a contour map. The contour map is then used to assess the defect, and calculate a safe operating pressure. All these manual measurement methods are laborious, time-consuming, and error prone.
There have been some devices that automate some aspects of inspections, using eddy current arrays, as, for example, in the following patents, which are incorporated herein by this reference: U.S. Pat. Nos. 5,793,206, 5,182,513, and 5,262,722. However, these all have various limitations, as set forth in U.S. Patent Application No. 20040232911.
U.S. Pat. No. 6,545,467, which is incorporated herein by this reference, states in the abstract, “A flexible eddy current array probe is attached to the contoured exterior surface of the backing piece such that the probe faces the contoured surface of the workpiece to be inspected when the backing piece is disposed adjacent to the workpiece. The backing piece is then expanded volumetrically by inserting at least one shim into a slot in the backing piece to provide sufficient contact pressure between the probe and the workpiece contoured surface to enable the inspection of the workpiece contoured surface to be performed.” However, the method disclosed in this patent is primarily concerned with ensuring coupling between a flexible eddy current array probe and a workpiece to be inspected. This patent does not disclose a two-dimensional eddy current array, does not disclose how to use such an array, does not provide the means to map defects, and does not use the flexible substrate to reestablish the contour of the workpiece.
Another device that uses eddy current arrays has been disclosed in the following patent applications, which are incorporated herein by this reference: U.S. Patent Application Nos. 20030164700 and 20040232911. These patent applications disclose a device that has essentially identical sensor arrays with sensing elements aligned in proximity to the drive elements, and conductive pathways that promote cancellation of undesired magnetic flux. These references do not disclose how to reestablish the original surface contour of a pipe, nor do they disclose any way to expand the dynamic range of the testing device.
Another device that uses eddy current arrays has been disclosed in the following patent application, which is incorporated herein by this reference: U.S. Patent Application No. 20060170420, which states in the abstract, “An eddy current testing probe has a flexible substrate adapted to face to a surface of a test article, a plurality of coils which are fixed to the flexible substrate and energized one of which is capable of being changed sequentially, a pressing member for pressing the substrate toward the test article, an elastic member arranged between the substrate and the pressing member, and a movement limiting member for limiting a movement of the pressing member toward the test article.” The device disclosed in this patent is again concerned about pressing an eddy current probe against a workpiece, or “test article”, and the attendant problems with exerting such pressure. The patent does not disclose how to map the dimensions of surface defects.
In light of the foregoing, a need remains for a flexible two-dimensional eddy current array that 1) provides a means to map defects on a pipe, 2) uses the flexible substrate to reestablish the original surface contour of a pipe, and 3) provides means to expand the dynamic range of the eddy current array.